Broadband absorber behind the speakers?

Hi, I try to do some concepts for my "student-living room-control room". The first thing I want to do is to cover all "first reflexion points" and add some basstraps (there is enough information about that in this forum) , but I also want to engage comb filtering and reflections from the wall behind my speakers (LEDE). So the idea is a broadband absorber between the speakers and the wall. (note: the picture is only a sketch) Because of my "all in one" room I can't sacrifice more then 30 cm for the absorber. I have done some simulations with this tool (http://www.whealy.com/acoustics/Porous.html): simulation at 0° with 703 (picture) http://img178.imageshack.us/img178/7787/703jp2.jpg simulation at 0° with rockwool 7000 rayls/m (picture) http://img165.imageshack.us/img165/7430/rockwoolaz8.jpg I have also tested at various angles, the difference between both of them remains constant. I know that a simulation is not reality, but the rockwool with 7000 rayls/m looks promising at ~30 cm. Futhermore I can get rockwool with 7000 ralys/m at a local store - the "703 equivalent" not. What do you think, a good idea or not? best regards JCR

JCR, Good to see you've done some home work. Depending on your room dimensions (I did not see them), you may not require a front wall absorber. In most cases, I have found them to be helpful, but only when the room is relatively small, and the panels are used sparingly. I am assuming you do not want to create a completely "dead" room, correct? This may not be case here. However, if you could offer some room dimensions, and better yet, some pictures of the space, we might be able to help you more. Cheers, Joel DuBay

JCR, > I also want to engage comb filtering and reflections from the wall behind my speakers (LEDE). < As I see it, that's the third priority for treating. Some people have dipole speakers that send sound equally from the front and rear, so in that special case absorption on the front wall can be useful. Other than that, the first important places are the first reflection points on the side walls and ceiling (and floor). Second in importance is the rear wall behind you, especially if it's less than ten feet away. Then you could consider the front wall. --Ethan

One of the other things you need to take into account that I don't think Chris' sheet does is the width and depth of the monitors - as well as the distance from the front baffle to the wall surface behind them. This will tell you what the SBIR response will look like - which in turn, can help you decide where you want to treat in the spectrum. Also, one must consider the same SBIR effects to the side wall boundaries. In some cases, you can get somewhat offsetting SBIR effects from the front and the side making the need to have really thick absorbers on the front wall unecessary. That said, I still prefer a dead front wall - though if you can get the rest dealt with in other ways, 2" is generally sufficient. Bryan

bpape

2" is generally sufficient

I think 1" is sufficient. Because the ITU standard is "reflections > 10dB less than direct sound in the 0 ms… 15 ms (in the region 1 kHz to 8 kHz)", and 1" will deal with 1khz. That said, I'm doing 7" on a front wall of mine. (Probably take me a month)

I'm doing 7" OK, the word's out - don't deny it, YOU are THE Bob in those ads for "Natural Male Enhancement" we see on late nite TV - I'm impressed... Steve

knightfly:

For purely reflections, yes, 1" would be fine - depending on what's going on in the rest of the room in terms of decay time balancing. Bryan

Bob, > I think 1" is sufficient. Because the ITU standard is "reflections > 10dB less than direct sound in the 0 ms… 15 ms (in the region 1 kHz to 8 kHz)", and 1" will deal with 1khz. < Yeah, but ... Forgetting front wall versus side walls etc, getting reflections down by 10 dB will reduce comb filtering peaks from a theoretical 6 dB to 2.4 dB and nulls from a theoretical depth of infinity to -4.3 dB. Personally, I'd rather have the peaks and nulls be as close to 0 as possible! Also, that's only for 1 KHz and higher. Don't you care about reducing the peaks and nulls below 1 KHz? I sure do! > That said, I'm doing 7" on a front wall of mine. < Exactly my point. So why would you tell others to do one thing and then do something different for yourself? :) --Ethan

Thanks for your answers, it's really great that there are so many helpful people out there ;-). Unfortunately I'am ill since yesterday, so I have to wait some days with this project... I will post an comprehensive answer when I'm well again. I hope this delay isn't a problem for you. best regards JCR

Ethan:

Exactly my point. So why would you tell others to do one thing and then do something different for yourself?

:) I'm on safer ground quoting the acoustic organizations who've reached conclusions based on decades of experience and more issues than I can name. Certainly getting rid of 30hz to 1khz modes is a good thing, but I'm not entirely convinced the front wall (< 15ms) is the best place to attack that. The room I'm doing that in is going to be an acoustical mess no matter what I do to the front wall. I'm in it now and it's an acoustical mess with lousy speach inteligibility, and a loud noise floor (fridge, air handlers). I can hear modes around 1khz without equipment (loud and quiet by moving my head 6"). I can't do any thing with the rest of the room. Basically my room is odd for a room here, but a typical asymetric living room. My left wall is a bookcase (nothing I can do about that, but it might give me some diffusion, and some back at the speakers reflectivity). My right wall is a hall with a wall parallel to the front wall that should eliminate right side reflection to at least one of the seats. The floor is carpeted. The speakers are a little higher than center room height, and toed downwards to reduce ceiling reflection. The 7" dimension was chosen not for an acoustic reason, but rather there's a 7" projection of the wall on one side, and there's a plasma on an arm that's about the same 7" out from the wall and I don't want to see the wires or the steel supports of the plasma tv. So I'm building a 6" deep false wall with 1" velcro GoM panels in front of it so that I can change the treatment behind it to anything anytime I want. (Also, so I can run wires) What I'm starting with is 2" rigid rockwool with a 4" airgap behind it. I might try 1" linacoustic against the wall as a second try. At 7", there's even an opporunity in there for poly's. Obviously you're warm when you're guessing that I'm trying to target frequncies lower than 1khz when I'm starting with 7" out from the wall. Because I am. :) That doesn't mean I'm certain it's a good practice. I not only can but actually will experiment, and I have some measuring tools (RplusD, calibrated microphone, microphone boom). I'm getting speakers that tested fairly well (you've seen the tests). If I end up with lots of spare rockwool, it just goes into the basement for other experiments (not wasted). I'd hate to 'recommend' something wrong to someone who wouldn't build something that they can change at a later date. I'm playing with acoustics as a minor hobby. I assume most people who start threads here are doing recording as some sort of business. Anyway, like I say a lot, "I give ideas not advice." And of course I'm maintaining the tradition of a) here's what the experts recommend, and b) but you can do whatever you want with your room, as long as you don't kidnap Floyd Toole and force him to listen to it.

Good points. As to the front wall per se, that's been debated many times. Personally, I think it's a great place to deal with length modes. Since bass radiates as a sphere, this is the first place that you can deal with it - before it has a chance to bounce back and cause SBIR issues. Bryan

Bob, > I'm on safer ground quoting the acoustic organizations who've reached conclusions based on decades of experience < Nah. :) Seriously, the deeper I get into this stuff, the more I realize how far off the mark many "experts" are about small room acoustics. I am not dissing education and research! But just because something appears in print doesn't make it so. I've seen plenty of blatant BS in the AES journal. I recall your friend Dan Nelson telling me in a phone conversation that some professional acousticians deny the usefulness of putting bass traps in the corners. Think about that! > Certainly getting rid of 30hz to 1khz modes is a good thing, but I'm not entirely convinced the front wall (< 15ms) is the best place to attack that. < Well, these are two related but slightly different things. One is early reflections and comb filtering, and the other is bass trapping. I agree with Bryan that the front (and rear) directions are the most important for bass trapping. So for bass trapping the length axial mode you want at least 4 inches thick if not 8 to 12. And for first reflections and non-bass comb filtering, reaching down to 300 Hz is important and 1-inch rigid fiberglass won't do that. I mean, at what frequencies do you not care about having a flat response? > The room I'm doing that in is going to be an acoustical mess no matter what I do to the front wall. < Knowing you, I am certain you'll end up with a stellar sounding room regardless. > Obviously you're warm when you're guessing that I'm trying to target frequncies lower than 1khz when I'm starting with 7" out from the wall. < :) > That doesn't mean I'm certain it's a good practice. < Why wouldn't it be good practice? --Ethan

Ethan wrote: >I've seen plenty of blatant BS in the AES journal. I recall your friend Dan Nelson telling me in a phone conversation that some professional acousticians deny the usefulness of putting bass traps in the corners. Think about that! < Did the person ever say why bass traps where not useful in a corner? That just seems nuts to me. Glenn

Ethan

Why wouldn't it be good practice

Lots of reasons. One reason - loading: a) As an alternative to front wall treatment we have baffling/soffeting -- which totally gets rid of front wall reflections, at the cost of 2pi loading (EQ or wall-mount speakers to fix). b) Low frequencies wrap around the speaker inversely proportional with frequency (lower frequency = more wrapping), and the reflection is part of the speakers' room performance/volume. Absorbing that LF with thick absorption may reduce the LF in the room giving a curve like Zaphod's http://img171.imageshack.us/img171/8720/clothfrsb6.gif . Or, more likely a dip in the response since even 7" of porous won't be terribly absorbant under 60hz. (In my case this is less of a problem because I'm doing 5.1 sound, and my subwoofer is my footstool, and the speakers close to the wall are crossed over at 80hz) c) thin absorption on the wall (1") doesn't suffer from what was described in (b). I like hearing bass. The best place for bass treatment is the other side of the room from where your speaker (subwoofer) is (e.g. if your subwoofer is in the front, then bass treatment in the back, perhaps as gung ho as "kill em all big and small"). That way the pressure has to go by you at least once, rather than having some of the life sucked out of it. Personally in the porous category I scoff at anything less than {6" of 703 with a 4" air gap behind it} as not much of a bass absorber. Even {6" of 703 with a 4" air gap behind it} I would bet wouldn't do much better than: 100hz 0.50 80hz 0.45 63hz 0.40 50hz 0.34 40hz 0.28 32hz 0.23 20hz 0.13 but at least it has the advantage of certainly not resonanting the wall nor ringing :) Glue it to the wall and it might even damp the wall. {2" of 703 with no air gap} does nothing (< 0.05) below 100hz. Although I never forget Terry Montlick mentioning some study that showed that 0.5" of material covering an entire wall had surprising bass absorption effects. I also have the option of changing the densities of porous material I use. More dense may have a flatter absorption curve (less in highs, more in bass), less dense the curve is more exponential, at least for E mounting. Covering an entire front wall certainly will reduce room effects (reverberation field vs direct), and have effects on sound path and spaciousness (especially wrecking music, but I'm after speech inteligibility first), etc. Minimalist panels (e.g. a horizontal porous stripe from 2' up the wall to 5' up the wall centered at ear hight) might be better than the entire wall. Kind of depends on how big the room is, and thus the sound path lengths (delays), as to what I'd put up there. Anyway, lots of possibilities other than a blanket "cover the entire front wall with deep absorption". In my case it might be good. I don't know. But it's what I'm building. :)

Personally in the porous category I scoff at anything less than {6" of 703 with a 4" air gap behind it} as not much of a bass absorber. Even {6" of 703 with a 4" air gap behind it} I would bet wouldn't do much better than: 100hz 0.50 80hz 0.45 63hz 0.40 50hz 0.34 40hz 0.28 32hz 0.23 20hz 0.13

What si the source for this data? The only testing I know of going down to 50 Hz shows the material having an absorption co-efficient equal to or greater than 1 down to 50 Hz! Curiusly: Andre

AVare:

What is the source for this data?

http://www.whealy.com/acoustics/Porous.html is where the numbers came from.

The only testing I know of going down to 50 Hz shows the material having an absorption co-efficient equal to or greater than 1 down to 50 Hz!

Which would that be? Do you mean 1.00 or 0.01 ? http://www.bobgolds.com/AbsorptionCoefficients.htm has some results down to 80hz and some others down to 50hz. http://www.rpginc.com/products/modexcorner/modc_imp.htm

AVare: As a double check, sometimes I do this sort of thing (assumes velocity is proportional to absorption, and sine is the velocity). Since the whealy numbers were based on 6" + 4" = 10", and the double check says it has to be 13" (13" = 1.1 feet) to get 0.13 at 20hz, the double check indicates that the whealy numbers are a little high, but in the ballpark. (Note I may have used the wrong rayls value in whealy.) The sine double check is usually pessimistic. Give it a try with some of the values on http://www.bobgolds.com/AbsorptionCoefficients.htm and you'll see what I mean. Probably due to diffuse sound field (sound coming in at other than normal incidence, wrecks the 1/4 wavelength equals depth asumption). Behind speakers 'diffuse' is a better model. At the back of the room, perhaps 'normal incidence' is a better model for early/first reflections. Hence my "I would bet wouldn't do much better than"

z60611 wrote:

AVare:

What is the source for this data?

http://www.whealy.com/acoustics/Porous.html is where the numbers came from.

So they came from the theoretical calculations for normal incidence. BTW I tried to d/l Chris' files and all I got was not available messages.

The only testing I know of going down to 50 Hz shows the material having an absorption co-efficient equal to or greater than 1 down to 50 Hz!

Which would that be? Do you mean 1.00 or 0.01 ?

I mean 1.00. The source is BBC RD 1992-10 fig 3, pdf page 8. The one that I am specifically referring to is is for the 280 mm thick material. This is from testing in a reverberation chamber. From what I recall from another BBC RD report, the normal incident absorption rolls off approximately an octave higher. The test data shows basically flat absorption down to 50 Hz, so a reasonable assumption is that the test material is flat down to at least 100 Hz for normal incidence. Obtusely: Andre

Bob, > As an alternative to front wall treatment we have baffling/soffeting < Well, okay, but now you're changing gears in the middle of the stream. Or something like that. :) > Low frequencies wrap around the speaker inversely proportional with frequency < All the more reason to pull a "Woodlock" and just fill the entire freakin' space with as much insulation as you can possibly fit. I say the heck with 1-inch thick anything. > In my case it might be good. I don't know. But it's what I'm building. :) < I'm sure it will be good. And I'm sure you'll have ETF data to prove it. --Ethan

Glenn, > Did the person ever say why bass traps where not useful in a corner? That just seems nuts to me. < Of course it's nuts. That what happens when you spend all your time in an ivory tower and don't get out enough. In this case it is incredibly easy to prove that corners are the single best place for bass traps. But some folks would rather argue (incorrect) theory than actually test it. Anyway, that was related to me in a conversation with Dan Nelson. I hadn't heard of that before, but I've heard it again since. I have no idea how a professional acoustician could not understand this, but recall it was only a few years ago that a number of "pros" argued vehemently with me that comb filtering peaks and nulls do not exist in a room at non-modal frequencies. :roll: --Ethan

Ethan Winer wrote:

Glenn, > Did the person ever say why bass traps where not useful in a corner? That just seems nuts to me. < Of course it's nuts. That what happens when you spend all your time in an ivory tower and don't get out enough. In this case it is incredibly easy to prove that corners are the single best place for bass traps. But some folks would rather argue (incorrect) theory than actually test it. Anyway, that was related to me in a conversation with Dan Nelson. I hadn't heard of that before, but I've heard it again since. I have no idea how a professional acoustician could not understand this, but recall it was only a few years ago that a number of "pros" argued vehemently with me that comb filtering peaks and nulls do not exist in a room at non-modal frequencies. :roll: --Ethan

Glenn, Ethan has his own manner to handle truth and context. Do you know that you call at this very moment Dr Peter D'Antonio and Prof. Dr. Trevor Cox nuts, because that possibly what Dan refers to. If you call something nuts, 1) you should know the arguments, 2) you should know and be able to explain what and why those arguments are wrong. Ethan, either refer to related posts of Dan Nelson, or ask him to bring his own words in context. Please tell why you questioned Scott Foster defending 703 mounted in the corners in favour of your panel traps. While for net visitors it feels that your "panels straddling the corner" where always there, we both know that this is all relative recent isn't it? RealTraps was established, not that long ago based on the production and sales of panel traps, not MiniTraps, not "panels straddling the corner". This kind of talk is children's talk. BTW: I never saw ANY PRO ACOUSTICIAN on the net tell that peaks and nulls do not exist at non-modal frequencies, which is quite different than telling that peaks and nulls do exist ALWAYS at ALL frequencies, which is what Ethan's point always was and which indeed is wrong. Ethan you really do believe yourself don't you? Interference is one of very first concepts anybody who learns acoustics learns about, which is handled in the first chapters of whatever related book.

AVare: I'm able to download http://www.whealy.com/acoustics/Porous Absorber Calculator V1.51 XL2002.zip

Ethan:

I say the heck with 1-inch thick anything.

We are of course free to do whatever we want in our own rooms. :) Another thought about 1" and 1khz is the way the ear works. Interaural time difference (ITD) works well up to about 743 hz. Interaural intensity difference (IID) works well above 2.8khz To say that again the interaural intensity difference is a queue for the direction at high frequencies whereas the interaural time difference is a cue for the direction at low frequencies. In between these two frequencies the ability of our ears to resolve direction is not as good as at other frequencies. For IID to work, reflections should be killed by 10dB. ITD is less sensitive to reflections, because it works relative to the time it takes sound to go around your head which is on the order of 673 microseconds (not milliseconds). Reflections off objects less than a head diameter away from the speaker cone should be avoided, but they'd be rare in most places, with the possible exception of a recording console with mini speakers sitting on the back of it.

All the more reason to pull a "Woodlock" and just fill the entire freakin' space with as much insulation as you can possibly fit.

I probably will. :) Maybe some rigid in the back, with SafeNSound in the front. Initially I thought "Pull a Woodlock" was a good definition and smiled with shared understanding -- but I recall that Paul Woodlock actually thought a lot about sound paths in his room, and there are reflective spots in chosen locations. So the criteria wasn't what "can possibly fit".

AVare:

The source is BBC RD 1992-10 fig 3, pdf page 8. The one that I am specifically referring to is is for the 280 mm thick material.

I like that. I think I'll add that to my http://www.bobgolds.com/AbsorptionCoefficients.htm page. BTW, when I was 8 years old and the newspaper reporter asked me what I wanted to be when I grew up I'm pretty sure I said "Fireman or Architect". I'm certain I didn't say "I want to collect fiberglass absorption coefficients." :) What a hobby. :o